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Lazy Load Your React Code With Code-Splitting

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React is a library for creating front-end views. It has a big ecosystem of libraries that work with it. Also, we can use it to enhance existing apps.

In this article, we’ll look at how to make apps load faster by splitting code so that only the parts that are needed will load.


Code-Splitting

We need code-splitting so that product React bundles won’t be too big. As our apps get bigger, the production bundles will also get bigger and take longer to load if we don’t split them and load them only when needed.

Create React App has code-splitting support built-in. We can use the lazy and import functions from React to achieve this.

For example, we can use those functions as follows:

Foo.js:

import React from "react";export default function Foo() {  
  return <div>foo</div>;  
}

App.js:

In the code above, we have Foo.js with the Foo component.

Then, in App.js, we have the App component that has Suspense to load the fallback. UI has the Foo import loads. We need this since Foo is loaded at runtime.

React.lazy loads the Foo component at runtime instead of at build time. It also splits the code from the bundle into a separate file so that each bundled file is smaller.

We’ll get an error telling us to add a fallback UI with the Suspense component if it’s missing.

The fallback prop accepts any React element that we want to render while waiting for the component to load. We can place Suspense anywhere above the lazy component.

Wrapping multiple elements in a single Suspense element also works.


Error Boundaries

To fail gracefully when other modules fail to load, such as where network failure occurs, we can use error boundary components.

To use them, we wrap them around the Suspense component as follows:

ErrorBoundary.js:

Foo.js:

import React from "react";

export default function Foo() {  
  return <div>foo</div>;  
}

App.js:

In the code above, we added the ErrorBoundary component in ErrorBoundary.js.

Then, in App, we wrapped it around our Suspense component, which lets us catch any errors that occur when loading the Foo component.


Route-Based Code-Splitting

Many React apps are single-page apps. They’ll have routes to map URLs to components.

We can split code based on routes. For example, we can incorporate React Router routes into our app and split code based on routes as follows:

Foo.js:

import React from "react";

export default function Foo() {  
  return <div>foo</div>;  
}

Bar.js:

import React from "react";

export default function Bar() {  
  return <div>bar</div>;  
}

App.js:

In the example above, we add the Router component around all our components.

We put the Routes inside the Suspense component so that they can be lazy-loaded. That is, they load only when we load the route in our browser.

This is also why we need the Suspense component around the routes. We need the fallback UI so that it’ll be shown when the routes are loading.


Named Exports

React.lazy only supports default exports. If our module uses named exports, then we have to create an intermediate module that re-exports it as default.

For example, we can arrange our code as follows:

Foo.js:

import React from "react";

export const Foo = function() {  
  return <div>foo</div>;  
};

FooDefault.js:

import { Foo } from "./Foo";

export default Foo;

App.js:

import React, { Suspense } from "react";
const FooComponent = React.lazy(() => import("./Foo"));
export default function App() {
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <FooComponent />
    </Suspense>
  );
}

In the code above, we have FooDefault.js that exports Foo as a default export.

Then, we imported it in App.js with React.lazy.


Conclusion

Code-splitting is easy with Create React App and React. React has the lazy method to import components on the fly.

We need a fallback UI so that we can see something when the dynamically imported component loads.

Error boundaries are also supported and we can use it to gracefully fail when a dynamically imported component fails to load.

Code-splitting is also supported by React Router. React.lazy is smart enough to split code by route.

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